Table 8.1 shows the percent of vulnerable CONUS land-mass over which the WSR-88D system is incapable of detecting various types of weather events. These percentages, taken from calculations performed for the 1995 National Research Council study, reveal that the WSR-88D is able to observe certain hazards in only 29-69 percent of vulnerable regions. The NWS does make use of TDWR and ASR data to address some of the gaps in populated regions where these radars provide additional coverage.

Westrick et al. (1999) assessed the impact of limited WSR-88D coverage for detection and quantitative estimation of precipitation amounts over the US west coast regions. This study concluded that, as a result of significant terrain blockage in that region combined with shallow depth of precipitation during cold seasons and low melting levels, 67-75 percent of the land surface in the region has inadequate radar coverage to support quantitative precipitation estimation.

Figure 8.1 shows the coverage provided by the combined WSR-88D, TDWR, ASR, and ARSR systems at 1,000 ft Above Ground Level (AGL). The white spaces reveal that the majority of the airspace at the 1000 ft level is not observed by these radar networks. This fundamental limitation of the ability of any widely-spaced network of ground-based radars to observe close to ground level results from both the curvature of the earth and blockage of the radar beam by mountainous terrain.

An MPAR network like that envisioned in the JAG/PARP report may be able to economically replace the current weather and aircraft surveillance system, and possibly enhance its capabilities while lowering life-cycle costs, but it will not be the entire national weather and aircraft surveillance solution. A number of DOD and DHS systems are currently used to help meet NAS surveillance requirements and likely will continue to be a key part of the NAS surveillance system. Weather surveillance is supplemented by a variety of independent radar and non-radar systems. Other new sensing systems which could address portions of the national surveillance needs are also being developed, including low-power, low-cost boundary layer radars (see Box 8.1) and acoustic and lidar systems.

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